According to Miller's Anesthesia, what are the indications and contraindications for spinal anesthesia?

Indications and Contraindications for Spinal Anesthesia

Spinal anesthesia is indicated for surgical procedures below the diaphragm lasting less than 3 hours, with absolute contraindications including patient refusal, infection at the puncture site, severe coagulopathy, and severe hypovolemia, while relative contraindications require careful risk-benefit assessment including anticoagulation status, bacteremia, and certain neurological conditions. 1, 2, 3

Indications

Spinal anesthesia is appropriate for:

• Surgical procedures below the diaphragm including lower abdominal, pelvic, perineal, and lower extremity surgeries 2, 3
• Lumbar spine procedures including microdiscectomies, laminectomies, and both single-level and multilevel fusions lasting less than 3 hours (the duration of intrathecal bupivacaine) 4
• Outpatient procedures when using small doses of long-acting agents like bupivacaine or ropivacaine 3
• Patients with significant comorbidities including obesity (BMI ≥30 kg/m²), anxiety, obstructive sleep apnea, and those undergoing reoperation at the same level, as these do not increase complication rates 4

Absolute Contraindications

The following are absolute contraindications where spinal anesthesia should not be performed:

• Patient refusal 1, 3
• Infection at the puncture site 1, 3
• Severe, uncorrected hypovolemia or shock 1, 3
• Severe coagulopathy with inability to achieve safe hemostatic parameters 1
• Increased intracranial pressure (except in pseudotumor cerebri) 3

Relative Contraindications Requiring Risk-Benefit Assessment

Anticoagulation and Antiplatelet Therapy

The timing of spinal anesthesia relative to anticoagulant administration is critical to prevent vertebral canal hematoma, with specific time intervals required based on drug pharmacokinetics. 1

Specific Drug Timing Requirements:

• Warfarin: INR must be ≤1.4 before performing spinal anesthesia 1
• LMWH prophylactic dosing: Wait 12 hours after last dose 1
• LMWH therapeutic dosing: Wait 24 hours after last dose 1
• Unfractionated heparin (subcutaneous prophylaxis): Wait 4 hours or until normal aPTT 1
• Clopidogrel, prasugrel: Wait 7 days after last dose 1
• Ticagrelor: Wait 5 days after last dose 1
• Rivaroxaban prophylaxis: Wait 18 hours after last dose 1
• Rivaroxaban treatment: Wait 48 hours after last dose 1
• Dabigatran (CrCl >80 mL/min): Wait 48 hours after last dose 1
• Apixaban prophylaxis: Wait 24-48 hours after last dose 1
• Thrombolytic drugs: Wait 10 days after last dose 1
• NSAIDs and aspirin: No additional precautions required 1

Inherited Bleeding Disorders

Specific factor levels must be achieved before spinal anesthesia in patients with inherited coagulation defects, with thresholds varying based on bleeding history severity. 1

Factor Level Requirements:

• Von Willebrand disease: VWF activity ≥50 IU/dL for mild bleeding history; ≥80 IU/dL for severe bleeding history 1
• Hemophilia A/B: Factor VIII or IX ≥50 IU/dL for mild bleeding history; ≥80 IU/dL for severe bleeding history 1
• Factor XIII deficiency: FXIII activity ≥50 IU/dL for mild bleeding history; ≥80 IU/dL for severe bleeding history 1
• Fibrinogen deficiency: Fibrinogen ≥1.5 g/L (Clauss method) for spinal anesthesia in mild bleeding history; ≥2.0 g/L for severe bleeding history 1

Bacteremia and Sepsis

Bacteremia is a relative contraindication requiring careful consideration, as the risk of CNS infection from spinal anesthesia during bacteremia is theoretical but not definitively proven. 5

• Antibiotic chemoprophylaxis should be administered before puncture if spinal anesthesia is performed during bacteremia 5
• Close postoperative monitoring for spinal epidural abscess development is mandatory 5
• Informed consent discussing the potentially increased risk of infectious complications must be obtained 5
• Active sepsis with hemodynamic instability remains a stronger contraindication 5

Thrombocytopenia

A platelet count of ≥75 × 10⁹/L is generally acceptable for spinal anesthesia in patients with platelet disorders and mild bleeding history. 1

• For patients with severe bleeding history, higher platelet counts may be required on a case-by-case basis 1
• The distinction between mild and severe bleeding is critical: mild bleeding has never required transfusion or hemostatic surgical intervention 1

Key Safety Considerations

Vertebral Canal Hematoma Risk

The incidence of vertebral canal hematoma after neuraxial blockade is extremely rare (0.85 per 100,000; 95% CI 0-1.8 per 100,000) in patients without coagulation abnormalities. 1

• Maintain high index of suspicion for signs of vertebral canal hematoma after spinal anesthesia, particularly in patients with any coagulopathy 1
• If block performance is traumatic, consider extending waiting times before catheter removal or subsequent anticoagulation 1

Common Pitfalls to Avoid

• Do not administer spinal anesthesia while newer anticoagulants (DOACs, fondaparinux) are at therapeutic levels - limited safety data exists 1
• Do not assume obesity, anxiety, or obstructive sleep apnea are contraindications - these patients do not experience higher complication rates 4
• Do not perform spinal anesthesia in patients on therapeutic LMWH within 24 hours - this significantly increases hematoma risk 1
• Do not overlook the need for multidisciplinary consultation in patients with inherited bleeding disorders and severe bleeding history 1